Matched-field replica models based on an inaccurate knowledge of
geoacoustic parameters such as bottom attenuation, shear, and interfacial
sound-speed discontinuities, can predict an incorrect number of propagating
modes for a shallow-water channel. The resulting degradation in the
matched-field ambiguity surface can be substantially reduced by optimizing over
the modal-sum-limit used in the replica model. The use of this technique for
multitone (70, 95, 145, and 195 Hz) source-tow data recorded near San Diego
during the first shallow water evaluation cell experiment (SWellEX-1)
significantly increased matched-field correlation levels for all narrow-band
tones (and their average), with the maximum peak more frequently coinciding the
correct source location than that observed with a previous nonoptimized model
[Schey and Ryan, 2981A (1994)]. The predicted number of propagating
modes was also reduced substantially. An inversion for bottom properties
(attenuation, interfacial sound-speed discontinuities, no shear) provided
sediment attenuation estimates which agree well with Hamilton's models and were
an order of magnitude greater than that used in the nonoptimized model, which
accounts for the reduction in the number of modes. A simulated model
decomposition using the new attenuation verifies the number of modes predicted
by the modal-sum-limit optimization.